Method for selecting and writing into RFID-transponders

ABSTRACT

Method and system for reading out and writing to RFID-transponders with inductive coupling including a read/write unit with a transmission frequency f s1  defined as a basic transmission frequency and a plurality of transponders operated at a resonance frequency f R . The resonance frequency f R  is fixedly predetermined, then for operating conditions with a high distance range, the resonance frequency f R  equal to the basic transmission frequency f s1  is selected. For operating conditions with a high recognition rate with a plurality of transponders located within a detection field, the basic transmission frequency f s1  is reduced to a fixed alternative value f s2  . This procedure guarantees reliable communication between the transponders and the read/write unit.

BACKGROUND OF THE INVENTION

The present invention relates to a method for reading out and writing toRFID-transponders with an inductive coupling, using a read/write unit,wherein the transponders operate at a resonance frequency. The inventionfurther relates to a system for reading out and writing toRFID-transponders, wherein the system comprises a transponder, which isdisposed on a carrier and has a resonant circuit with a resonancefrequency, a receiving unit and a read/write unit with atransmitter/receiver.

RFID-systems are becoming increasingly more frequently used for means ofcontact-free, automatic identification purposes. Approx. 90% of all soldRFID-systems are nowadays inductively-coupled systems with an inductivecoupling between the reading device and the transponder. Such systems,so-called remote-coupling systems generally function in the ranges of upto 1m in the read/write operation.

The transmission frequencies used are frequencies below 135 kHz or thefrequencies 6.78 MHz, 13.56 MHz and 27.125 MHz, i.e. the ISM frequencyranges which are held free especially for industrial, scientific ormedical purposes. Depending upon the frequencies used, differences occurin the data transmission rates, clock frequencies, output, etc.

It is possible nowadays to use RFID-labels to identify goods and otherobjects as these labels can now be produced practically as thin asconventional adhesive labels and thus are not generally recognised bythe user as being RFID-labels. For example, RFID-labels can be adheredor laminated to books, periodicals or similar documents.

RFID-systems which function in the radio frequency range (3 MHz to 30MHz) operate with LC-resonant circuits at a resonance frequency f_(R).If a magnetic alternating field acts with a frequency f_(s) on thetransponder of an RFID-label, then the resonant circuit of thetransponder starts to respond and is excited to resonance oscillation.In so doing it takes energy from the magnetic alternating field, whichfor example can be acquired by increasing the coil current or thevoltage drop at the internal resistor in the transmitter circuit. Inthis manner the operating voltage for the transponder chip can also beproduced.

In the case of EAS-systems, i.e. electronic article security, a warblefrequency is used. The transmitter frequency continuously scans afrequency range. This can be recognised by the energy absorption whichoccurs at an unknown resonance frequency of a transponder. DE 195 14 601A1 describes an EAS-system of this type which has a broadbandpre-amplifier which for example in the case of two transponder typespasses through their two frequency ranges one after the other.

The drop in voltage in the transmitter circuit as a consequence of thereceiving transponder being excited to oscillate is exploited duringload modulation, when by means of switching on and off the load resistorof the transponder voltage changes are caused at the antenna of thetransmitter and thus amplitude modulation of the antenna voltage iseffected.

If two RFID-labels are located in close proximity to each other,possibly stacked one on top of the other in a document file or adjacentto each other on a bookshelf, then these have a mutual effect on eachother during reception, i.e. they receive approximately with identicalstrength an in-phase signal from the transmitter and coupling effectsoccur. If they lie precisely one above the other, then there ispractically a common coil, wherein the two capacitors are connected inparallel. Thus a frequency displacement occurs, i.e. a change in theresonance frequency. This leads to the relevant read device only beingable to receive the data in a limited form or not being able to receivethe data at all.

Tests have shown that the resonance frequency of an RFID-label alwaysdisplaces downwards when a second RFID-label comes into the couplingrange of the first. In the extreme case, a rigid coupling can occur,wherein the resonance frequency of the two RFID-labels then amounts to

f _(R)/{square root over (2)}

Even when using high field strengths, i.e. high transmitter output, itis not always possible to communicate at the transmission frequencyf_(s). Depending upon the coupling level and resonant frequency of thetransponder circuits, zero settings can lie above the resonancefrequency which has been displaced downwards. If such a zero settingoccurs particularly where the transmission frequency f_(s) is used, thenthe chip no longer functions.

This is illustrated with reference to the replacement circuit diagramsin FIG. 4. There is located in the transmitter branch of thetransmitter/receiver A an oscillator 2 with a frequency f_(s), whoseoutput signal is transmitted, where appropriate after modulation, to anoutput end phase 4. The receiving branch which commences immediately atthe antenna bush is provided with a demodulator 6 and a band pass filter8 or a different filter. The antenna is a coil 10 with inductivityL_(s). The replacement circuit diagrams of two RFID-labels a,b are alsoillustrated. These include in each case a coil 12 with inductivity L₁and L₂ and a capacitance 14 parallel to the transponder chip 16. Thecoupling ratios are illustrated by lines, wherein k_(s) represents thecoupling transmitter/receiver and the transponder and k represents thecoupling between the two transponders. If there are differences in size(and thus coupling) or frequency of the RFID-labels, then the mentioneddeletions occur.

FIGS. 5 and 6 illustrate simulations of this process of the undesiredcoupling between two adjacent RFID-labels with like and unlike resonancefrequency.

In the first example (FIG. 5) the frequency responses for transponderswith identical resonance frequency are illustrated, where there is nocoupling (k=0) at 13.56 MHz and total coupling (k=1) at approx. 10 MHz.Thus, as the coupling increases the occurring resonance frequency dropsto lower frequencies.

In the case of different resonance frequencies as shown by the secondexample illustrated in FIG. 6, there is in fact again no coupling (k=0)at 13.56 MHz and the coupling increases [sic] to lower frequencies,wherein again total coupling (k=1) occurs at approx. 10 MHz. However, azero setting x has formed just above 13.56 MHz, so that under certainconditions the chip can fail.

It is known to use RFID-systems which operate in the microwave range toeliminate randomly occurring interference signals, where thetransponders operate at several resonance frequencies. This is the casewith an RFID-system described in U.S. Pat. No. 5,446,447 for reducingthe read time.

Furthermore, RFID-systems with transmission frequencies in the microwaverange have been used, where the transmission signal is modulated with asignal of e.g. 1 kHz and in addition to the resonance frequency thesecond harmonic of the transponder is also detected. Followingdemodulation and the passing through of a 1 kHz detector it is possiblein a reliable manner to distinguish between the received transpondersignals and interference signals and thus false alarms are avoided.However, disadvantages of these RFID-systems are the influences exertedby the multi-path and running-time effects.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method which renders itpossible to perform a reliable operation, in particular communicationand antenna feeding, even if several transponders are located over alarge spatial area.

Thus, in the case of the method in accordance with the invention forreading out and writing to RFID-transponders with inductive couplingusing a read/write unit, the transponders operate at a fixed resonancefrequency. The transmission frequency is reduced from a transmissionbasic frequency according to this resonance frequency for operatingconditions at a high range to a fixed alternative value of thetransmission frequency for operating conditions with a high recognitionrate, so that a reliable communication between transponder andread/write unit is guaranteed.

The method in accordance with the invention is suitable for use in therange of approx. 10 kHz to approx. 30 MHz. It is preferably used in theradio frequency range. In preference, the following values are providedas frequency combinations:

a) f_(R)=13.56 MHz; f_(s1)=13.56 MHz, f_(s2)=6.78 MHz

b) f_(R)=27,125 MHz; f_(s1)=27,125 MHz, f_(s2)=13.56 MHz, f_(s3)=6.78MHz

The manufacturing tolerances allow the transponder resonance frequencyto be varied by approx.±2%.

By providing two operating frequencies it is possible on the one hand toeliminate interference caused by other transponders located in theproximity or in any case to reduce it greatly and on the other hand tooperate with the necessary range. In an advantageous manner the resonantcircuits of the transponders are adjusted so that they individuallyoperate to an optimum on the transmission basic frequency and also havethe greatest range there.

If it is to be assumed that in the proximity of a transponder there areother transponders whose signals could interfere with the communication,the operating frequency, i.e. the transmission frequency of thetransmit/read unit, is reduced from about 13.56 MHz to the (or a) lowervalue 6.78 MHz which thus lies further away from the resonance frequency13.56 MHz of the transponder. As a consequence, the likelihood ofinterference is reduced and the recognition rate is increased, as isnecessary for densely packed RFID labels. It is thus possible in thecase of differing inquiry frequency and resonance frequency to detectwith a reliable recognition rate many transponders with the sameresonance frequency simultaneously in the read field. The transmissionrange is automatically lower in the case of a reduced transmissionfrequency. If, on the other hand, a higher range (recognition distance)of the transmitter is required and less interference is anticipated,then the higher transmission frequency is set. The method in accordancewith the invention thus provides for the same transponders to operatewith different transmission devices irrespective of the on-siteconditions.

Thus, for example, a gate system at the exit of a library, a warehouseor a sales outlet can transmit at the basic frequency (e.g. 13.56 MHz),since little interference from other transponders is to be expected atthis location. On the contrary, the transmission range here is greater.However, the read/write units of the warehouse and sales managementoperate at a lower alternative transmission frequency (e.g. 6.78 MHz) ashere the range is not so important, but extensive protection againstinterference is.

The system in accordance with the invention for reading out and writingto RFID-transponders which is suitable in particular for implementingthe method in accordance with the invention comprises a transponderwhich is disposed on a carrier and has a resonant circuit with aresonance frequency, a receiving unit and a read/write unit with atransmitter/receiver, wherein the resonant circuit has a fixed resonancefrequency and the transmitter/receiver of the read/write unit has alower transmission frequency (transmission frequencies) according to theresonance frequencies of the transponder as an alternative to the basictransmission frequency.

In so doing, transmitters having several adjustable transmissionfrequencies, for example 13.56 MHz and 6.78 MHz can be used in manyways.

The system in accordance with the invention can preferably be used withextremely thin labels which are embedded in the most varied of goods.

The method and system in accordance with the invention for reading outand writing to RFID-transponders can be used as a replacement foroptical barcode systems (retail, logistics, warehouse management); as areplacement for SmartCards (payment cards, guarantee cards, discountcards); as means of identification (books, documents, passes, tickets,certificates); to ensure copyright protection (books, dresses, soundmedia). This list is merely an example and is by no means complete. Incomparison to the conventional systems, when using RFID-transponders thesignificant advantages are a high read rate and the lack of dependencyon the positioning, meteorological conditions and the absence of wear.One or more transponders can be supplied with sufficient operatingvoltage, so that these can become active and can produce a response bymeans of load modulation.

This response is transmitted synchronously with respect to the carriersignal but on an integral ratio thereto, wherein the clock in thetransponder chip is obtained by separation from the transmission signal.In one embodiment an amplitude-modulated auxiliary carrier with afrequency f_(s)/32 is used as the return channel, i.e. the returnchannel is on f_(s)=f_(s)±f_(s)/32.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail hereinunder with reference topreferred exemplified embodiments, wherein the illustration servesmerely for illustration purposes and should not limit the invention tothe feature combinations illustrated. In the drawing:

FIG. 1 shows a block diagram of a read/write circuit in accordance withthe invention,

FIG. 2 shows an example for the use of RFID-labels in accordance withinvention in a bookshelf,

FIG. 3 shows an example for use of RFID-labels in accordance with theinvention in a document file,

FIG. 4 shows replacement circuit diagrams of transmitter/receiver andtransponder circuit for illustrating the coupling of two adjacenttransponders,

FIG. 5 shows an illustration of the simulation of the process ofundesired coupling between two adjacent RFID-labels with identicalresonance frequency, and

FIG. 6 shows an illustration of the simulation of the process ofundesired coupling between two adjacent RFID-labels with differentresonance frequencies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is described hereinunder with reference to an exemplifiedembodiment of a read/write circuit which is illustrated in FIG. 1. Thefirst to be described is the structure of part (a) of the read/writeunit. A voltage supply 42 can be connected to the supply network oralternatively can be provided via a battery or accumulator. In theillustrated exemplified embodiment the voltage supply 42 is connected tothe supply network with 230 V and the supply voltage can be measured atthe output 44.

The transmission branch of the read/write circuit comprises anoscillator (quartz oscillator) 22 with four outputs f(1), f(2), f(3),f(4) for four transmission frequencies. A selection switch 24 controlledby a micro-controller 30 is connected at the input side in each case toa selected one of the four outputs, in this case output f(1) of theoscillator 22 and at the output side to a modulator 26. The carriersignals transmitted by the oscillator 22 with the transmit commands anddata are modulated in the modulator 26. An output amplifier 28 isconnected to the output of the modulator 26 and is connected to themicro-controller 30 which controls its output power. The transmit signalamplified in the output amplifier 28 is passed via a line (coaxial linewith 50 Ω impedance) 50 to an antenna adaption circuit which comprisestwo variable capacitors 51, 52. The capacitors 51, 52 must possibly betuned according to the transmit frequency in order to be able to performthe required impedance transformation, wherein the dimensioning of thecapacitance values is preferably controlled via the micro-controller 30.The transmit signal is then emitted via an antenna 54.

The receiving branch begins at the nodal point 31, where the voltagevariation induced into the antenna 54 is measured. A demodulatorcomprises a peak value rectifier 32 and a threshold switch 34 and isconnected to the micro-controller 30 which processes the demodulatedsignals. An optointerface 40 is connected to the output of themicro-controller 30. A data transfer to a control computer can also beprovided, wherein commands and data can also be input into themicro-controller 30 via the same line, as indicated by the double arrow(DATA).

FIG. 1 further illustrates at (b) the part of the transponder circuit.This comprises an antenna 60 with a capacitor 62. A full wave rectifier64 rectifies the voltage induced by the read/write unit, i.e. themagnetic alternating field emitted by the antenna 54. The signals whichit emits are directed to the transponder chip 66. When the transponder66 emits a message to the read/write unit, the associated data aredigitally coded via a parallel-connected series circuit of the fieldeffect transistor 68 and resistor 70 via an auxiliary carrier, in thatthe resistor 70 is switched on and off via the field effect transistor68.

A circuit as shown in FIG. 1 (a) can be provided for example both foruse in a gate and also in a warehouse or sales room. In the gate, wherelittle interference from other adjacent transponders is to be expected,in order to achieve a higher range of the transmission signal thefrequency f(1) (namely in the present case 13.56 MHz) is used. On theother hand, the circuit for a device in a sales room is operated at alower frequency f(2) (namely in the present case 6.78 MHz), where thefields of closely disposed transponders do not cause interference.

FIG. 2 illustrates an example for the use of the method in accordancewith the invention in the case of RFID-labels for books. By way ofexample, RFID-labels 72 with a resonance frequency 13.56 MHz are adheredto the inside of the covers of books 70 which stand on a shelf [notillustrated]. By reason of the uniform manner in which the RFID-labels72 are attached they are more or less in alignment with each other sothat their coils are essentially one on top of the other and theyfunction as a common coil. The flow of the magnetic field and thethrough-flow of the transmission and transponder antennae is illustratedby H. In the case of a transmission frequency of 13.56 MHz considerableinterference would occur until the transponder signals disappeared. Byusing a transmission device at 6.78 MHz it is possible for thetransponder signals to be received in a reliable manner.

A similar case arises in the example illustrated in FIG. 3 with adocument file. The RFID-labels 72 with a resonance frequency 13.56 MHzare attached in each case to a comer region of a document 74 andlikewise cover each other, so that in the case of a transmissionfrequency of 13.56 MHz considerable interference would occur (see alsomagnetic field H). Also here a transmission frequency of 6.78 MHz isadvantageously used.

What is claimed is:
 1. Method for reading out and writing toRFID-transponders with inductive coupling comprising a read/write unitwith a transmission frequency f_(sl) defined as a basic transmissionfrequency and a plurality of transponders operated at a resonancefrequency f_(R), comprising the steps of: fixedly predetermining theresonance frequency f_(R), for operating conditions with a high distancerange, selecting the resonance frequency f_(R) equal to the basictransmission frequency f_(s1), and for operating conditions with a highrecognition rate with a plurality of transponders located within adetection field, reducing the basic transmission frequency f_(s1) to afixed alternative value f_(s2), thereby guaranteeing reliablecommunication between the transponders and the read/write unit. 2.Method according to claim 1, wherein the alternative value f_(s2) forthe transmission frequency is half of the basic transmission frequencyf_(s1).
 3. Method according to claim 1, wherein several alternativetransmission frequencies below the basic transmission frequency f_(s1)are used.
 4. Method according to claim 1, wherein the resonancefrequency is 13.56 MHz and as transmission frequencies are 13.56 MHz and6.78 MHz.
 5. Method according to claim 1, wherein the resonancefrequency is 27.125 MHz and the transmission frequencies are 27.125 MHz,13.56 MHz and 6.78 MHz.
 6. System for reading out and writing toinductively coupled RFID-transponders for implementing the method inaccordance with claim 1, comprising: a transponder disposed on a carrierand having a resonance circuit with a resonance frequency f_(R), areceiving unit, and a read/write unit with a transmitter/receiver andtransmission frequency f_(s1) defined as a basic transmission frequency,wherein the resonance circuit has a fixed resonance frequency f_(R) andthe transmitter/receiver of the read/write unit has a transmission basicfrequency f_(s1) equal to the resonance frequency f_(R) of thetransponder and a fixed alternative lower transmission frequency f_(s2).7. System according to claim 6, wherein the transmitter/receiver isadjustable to transmit a plurality of alternative lower transmissionfrequencies.